Various types of electromagnetically actuated clutches for transmitting rotary motion are known in the art. In one type of magnetically actuated clutch, disclosed, for example, in Lowery et al U.S. Pat. No. 4,030,584 and Baer U.S. Pat. No. 3,349,880, a helical clutch spring carried by an input or driver clutch member with portions loosely surrounding an output or driven clutch member has its free end attached to the armature of a magnetic circuit. Energization of an annular coil surrounding the clutch assembly pulls the armature against a flange on the output member in Lowery et al and against a stationary friction surface in Baer to retard the armature to cause the spring to wrap down on and engage the output member. Owing to the complexity of construction of clutches of this type, they are relatively expensive.
In another type of magnetically actuated clutch known in the art, such as disclosed in Wahlstedt et al U.S. Pat. No. 3,974,902, Baer et al U.S. Pat. No. 3,682,622, or Mason U.S. Pat. No. 3,177,995, the free end of the clutch spring, which is formed of magnetic material, is disposed in the gap of a radially symmetric magnetic circuit controlled by an annular coil surrounding the clutch assembly. Energization of the magnetic coil draws the free end of the clutch spring toward a relatively moving magnetic core portion to cause the spring to wrap down on the other clutch member. While clutches of this type do not require separate armatures such as described above, their magnetic circuits nevertheless add appreciably to the bulk and complexity of the overall clutch assembly. In addition, these clutches require a clutch spring formed of a magnetically permeable material.